Normally wikitext like {{subst:a|b}} is not saved as such if the template exists, because the substitution is applied. It can be created using Template:Timc by {{subst:subst}}a|b}} producing the wikitext {{subst:a|b}}; on the next save (with or without edits elsewhere on the page) this results in substitution of template "a".

However, in {{subst:subst}}a|b}} the visible braces are not balanced. If the whole code is nested inside another pair of double braces, the final closing braces of the inner code are taken as closing braces of the outer pair, e.g., using Template:Tim:

However, the first pipe in the inner code is not inside one of the pairs of braces in the inner code, so it is taken as parameter separator of the outer template or parser function. Therefore we also need Template:Tim:

The resulting page can have the same property of making a saved page containing subst, if we use a template "a" specially designed for this purpose.

To make this template static, <includeonly></includeonly> is put before (or around) every "subst:".

A basic form is that of Template:Timc where b represents the parameters, a the output before the template call, and c the output after the template call. These can all depend on the parameters. Starting with the wikitext {{subst:tsubst|..}}, on every save "a" is added before the braces, the parameter values are replaced, and "c" is added after the braces.

The wikitexts "a", "b" and "c" can contain {{<includeonly></includeonly>subst:..}}, in which case the wikitext produced is the result of expansion of the variable, parser function, or template at the time of saving. Also, ~<includeonly></includeonly>~~ gives the signature of the user.

Thus "b" represents the latest situation, and "a" and "c" the history, where chronological order is inward, toward the template call; in other words, the history grows from the inside.

If it is so desired, it may be set up so that the template stops reproducing under a certain condition (e.g. by substituting an empty page), only leaving the old versions of "a" and "c" behind, and, if any, the result of the last substitution in between.

If in a substitution a template calls itself, we may get the message "template loop detected" as wikitext comment. Also hidden braces can cause wrong parsing. Sometimes this is harmless because there is a next phase in which the parsing is correct. Templates like Template:Tim, Template:Tim, Template:Tim and, in general, these templates help to get it all correct. See also Template:Tim.

Instead of putting the construct of the previous section inside an if-construct, we can also have conditions inside it, e.g. the name of the template and each parameter can depend on conditions. See e.g. Template:Tim.

ParserFunctions has the odd limitation that not all numbers produced as output are accepted as input (see also Template:Mlmw, where this has been rectified): numbers with absolute value outside the range 1e-4 until 1e12 (for Wikimedia; it depends on the operating system of the server) are produced in scientific notation, but only accepted in regular notation.

While this does not affect intermediary results within an expression evaluated by a single #expr or #ifexpr, it affects intermediary results in the case of nesting of such functions, hence also intermediary results of a substitution template call, even if that call carries out multiple iteration steps in one save.

Any iterative process where a parameter converges to a value outside or near an edge of the range 1e-4 until 1e12 is likely to have intermediary results outside this range, after which no further computations are possible. Rescaling can help to carry out the computation anyway.

<span style="display:none">..</span> can be used to hide the template call. Proper nesting inside or around pairs of double or triple braces is not needed, during template expansion these tags are treated as plain text. Thus e.g. the opening tag can be in "b" and the closing tag in "c".

If we have a non-empty "c", for hiding the closing braces "c" needs to start with "</span>". Opening and closing span tags have to be balanced to avoid that the template call and/or its output affect the style after that. This requires an opening span tag in "c" to match the closing tag in the "c" that was previously put. It can come either immediately after the closing tag at the start of "c" (optionally specifying a style overriding the external style), or at the end of "c" (the external style applies also for "c"). If we have a non-empty "a", for hiding the opening braces "a" needs to end with "<span style="display:none">". Thus "a" has to start with a closing span tag to match the opening span tag in the previous "a" (the external style applies for "a", unless it contains an extra pair of span tags). Finally, to match the closing span tag at the start of the leftmost (oldest) "a" an opening span tag is needed to the left of that, while to match the opening span tag in the rightmost (oldest) "c" a closing span tag is needed to the right of that. If "a" is empty this opening span tag is also put; similarly, if "c" is empty this closing span tag is also put.

Thus the whole template call, including "b", is hidden, unless inside "b" we have "</span>" and "<span style="display:none">"; in that case the part of "b" in between has the external style, unless it contains an extra pair of span tags.

Dummy span tag pairs (either with nothing in between or without style specification) could be cleaned up manually from time to time.

In the case of automatic conversion of wikitext the original wikitext is not automatically documented. The edit summary can be useful for that. In addition, in the case of automatic conversion by just pressing edit and save, both the wikitext before and after the conversion are in the edit history. A code could be put in the edit summary to indicate "no manual change in the wikitext". Thus only the wikitext entered for the first step is undocumented, unless it is put in the edit summary (whose capacity may be too small), or is put in pre- or nowiki-tags in the page itself or the talk page.

Using a two-step method like {{subst:iterate|5|{{subst:square root|5|2}}}} we get in the edit history {{subst:#if:x|{{subst:#if:x|{{subst:#if:x|{{subst:#if:x|{{subst:#if:x|{{subst:square root|5|2.25}}<br>2}}}}}}}}}}, providing in this case a fairly good documentation:

the 5 is constant

the starting value 2 is in this case documented by producing it as first output.

Overview of various levels of substitution obtained by putting the call to the template in one or more levels of parser functions (which for larger numbers of levels can be abbreviated using Template:Tim):

The following output is the result of the above plus subsequent edit/saves:

1 steps with 1 edit:

aaaaaa{{subst:tsubst|b}}cccccc

2 steps with 1 edit:

aaaaaaa{{subst:tsubst|b}}ccccccc

3 steps with 1 edit:

aaaaaaaa{{subst:tsubst|b}}cccccccc

aaaaaaaa{{subst:tsubst|b}}cccccccc

4 steps with 1 edit:

aaaaaaaaa{{subst:tsubst|b}}ccccccccc

aaaaaaaaa{{subst:tsubst|b}}ccccccccc

2 steps with 2 edits:

aaaaaa{{subst:tsubst|b}}cccccc (see above)

3 steps with 2 edits:

aaaaaaa{{subst:tsubst|b}}ccccccc (see above)

aaaaaaa{{subst:tsubst|b}}ccccccc

documented by aa{{subst:tsubst|b}}cc in the wikitext after 1 edit

4 steps with 2 edits:

aaaaaaaa{{subst:tsubst|b}}cccccccc

documented by aaa{{subst:tsubst|b}}ccc in the wikitext after 1 edit

aaaaaaaa{{subst:tsubst|b}}cccccccc (see above)

aaaaaaaa{{subst:tsubst|b}}cccccccc (see above)

5 steps with 2 edits:

aaaaaaaaa{{subst:tsubst|b}}ccccccccc

documented by aaaa{{subst:tsubst|b}}cccc in the wikitext after 1 edit

aaaaaaaaa{{subst:tsubst|b}}ccccccccc (see above)

aaaaaaaaa{{subst:tsubst|b}}ccccccccc (see above)

Examples of templates providing an arbitrary number of levels of substitution:

Template:Tim - calculates, with Newton's method, the square root of parameter 1, with initial guess being parameter 2, with absolute error stop criterion (terminating when two successive estimates are within parameter 3), in two saves (or, with a parser function, in one save), printing all successive estimates and the differences between consecutive ones